Low pressure ultraviolet lamp

ABSTRACT

A low pressure ultraviolet lamp with an energy output effective in polymeric resin curing comprises a fused quartz envelope and a fill of mercury, argon and neon.

United States Patent Roche LOW PRESSURE ULTRAVIOLET LAMP William J.Roche, Merrimac, Mass.

GTE Sylvania Incorporated, Danvers, Mass.

Filed: Dec. 27, 1972 App]. No.: 318,827

Inventor:

Assignee:

U.S. Cl. 313/225, 313/185 Int. Cl. H0lj 61/12 Field of Search 313/225,185

References Cited UNITED STATES PATENTS 12/1939 Alterthum et a1. 313/225June 25, 1974 12/1939 Alterthum et a1. 313/225 8/1954 Heine et al.313/225 Primary Examiner.10hn Kominski Assistant Examiner-Darwin R.Hostetter Attorney, Agent, or Firm-James H. Grover ABSTRACT A lowpressure ultraviolet lamp with an energy output effective in polymericresin curing comprises a fused quartz envelope and a fill of mercury,argon and neon.

3 Claims, 1 Drawing Figure LOW PRESSU ULTRAVIOLET LAMP BACKGROUND OF THEINVENTION Low, medium and high pressure ultraviolet (UV) lamps, known tobe useful bactericidally, are typically manufactured with an elongatetubular envelope of silica glass or fused quartz with a fill or argongas and mercury. The gas fill pressure of low pressure lamps is at orbelow approximately 7 torr, of medium pressure lamps up to about 100torr, and of high pressure lamps one to several hundred torr. At lowpressure the UV output is largely (e.g. 90 percent) at the 2537 A(angstrom) line. Increasing fill pressure shifts the UV output towardlonger wavelengths and reduces each line to very low proportion (e. g.about l0 percent) of the UV energy.

In addition to bactericidal uses an important industrial application ofultraviolet lamps is in the photopolymeric curing of solventless inksfor high speed printers. Such inks, instead of a relatively slow dryingsolvent, contain a dry resin which cures and sets, photopolymerically onexposure to light. Practically, UV light is used, infrared light havinga heating effect strong enough to damage the printing paper and ink.Further, suitable resins are selectively responsive to particularwavelengths. A lamp providing a suitable UV output must also have a longlived continued output near its initial energy maximum to insure uniformphotopolymeric curing. With high speed presses running 500 to 1,000sheets per minute, the lamp cannot extinguish for more than a fewseconds due to momentary power interruption or reduction without aprohibitive loss of uncured sheets. The lamps must restart readily,rapidly and reliably.

The above requirements are not met by existing UV lamps in any of thethree pressure ranges.

Prior low pressure lamps with a fill of pure argon gas have too low a UVenergy output for curing, and the energy drops off rapidly in the firstfew hundred operating hours. They also are too slow in restarting to bereliable.

Medium and high pressure UV lamps are much too slow in restarting. Inlamps of comparable power their spectral energy is spread over severalrelatively weak wavelengths some of which are ineffective to curewavelength-selective resins. Thus although medium and high pressure UVlamps have a relatively higher energy output than low pressure lamp, alarge portion of the energy may be useless for curing a particularresin. And the higher energy output also includes more infrared energyin proportion to useful UV, thereby creating a heating problem. Furtherthe high energy output is provided by inordinately higher power inputrequirements and reactive lamp ballasts of weight and expense out ofproportion to that of low pressure lamps.

Accordingly the object of the present invention is to provide a lowpressure UV lamp having low power input and ballast requirements, whichemits substantially greater UV energy at a useful wavelength remote fromdaylight and infrared wave-lengths than prior low pressure lamps, whichis long lived and fast in restarting, and which is therefore highlypractical for use in polymeric curing.

STATEMENT OF INVENTION According to the invention a low pressureultraviolet lamp achieving the above stated objects comprises a fusedquartz envelope, electron emissive electrodes spaced apart within theenvelope for supporting an arc discharge therein, and a fill in theenvelope of mercury and a mixture of neon gas and between 0.l percentand 10 percent by volume of argon gas at a pressure of ap- O proximatelyseven torr or less.

DRAWING The single FIGURE is an elevation of a low pressure ultravioletlamp shown partly in section.

DESCRIPTION The low pressure, high intensity ultraviolet lamp shown inthe FIGURE comprises an envelope 1 of fused quartz tubing with, forexample, standard T8 dimensions of 1.5 mm. wall thickness, 1 inchoutside diameter, and length from a few inches to several feet. Unlikeprevious low pressure UV lamps the envelope has a press 2 at each endand a side exhaust part 3 which reduce the cost of assembly. Molybdenumleadin wires 4 are welded to molybdenum ribbons 6 within the press 2.Molybdenum wires 7 welded to the ribbons 6 and spaced by a glassinsulator 8 extend inwardly to nickel wires 9 forming part of thesupport structure for one of two electron emissively coated, coiledelectrodes 11. The support wires 9 are spaced by a ceramic insulator 12and hold a disk-shaped heat shield 13 with a feed through insulator 14and also a rectangular anode 16.

After exhausting and prior to scaling the envelope 1 was filled with 40milligrams of mercury in a 30 inch lamp and a gas fill of neon and argonat a pressure of between 0.1 and 7 torr.

I have discovered that with a fill of neon gas and the balance ofbetween 0.1 and 10 percent by volume of argon gas a low pressure lampwill operate with a higher input wattage and a markedly higher UV energyoutput than conventional low pressure UV lamps. A fill of less than 0.1percent argon does not strike an arc reliably. Above 10 percent the lamptends to saturate in operation at about 1 to 2 watts per inch powerdensity in a T8 envelope and is limited in energy output to about 65milliwatts of UV energy per square inch at 1 meter, a level quiteimpractical for photopolymeric curing. In contrast a fill of 0.1 to 10percent argon, preferably 0.5 percent, and at 2.5 to 3 torr will notsaturate below 9 watts per inch and will emit several times the UVenergy, e.g., 416 milliwatts per inch at 1 meter. At or somewhat below 7torr, emission at 2537A decreases without a compensating improvement inlamp maintenance. This marked increase in output in a low pressure lampis not predictable from performance of medium and high pressure lampsbecause of the effect of pressure on spectral distribution, and iswholly effective at one meter or less for photopolymeric curing.

In cooperation with the higher wattage input capacity of the presentlamp its quartz envelope affords continued efficient radiation of theemitted UV in a way unrelated to the small difference of UVtransmissivity of quartz as compared with silica glass. Quartz can beexpected to transmit roughly 10 percent more UV than silica glass atprevious wattage input densities. But at the several-fold higher wattageinput of the present lamp silica glass discolors at such a rate thatexternal energy output drops off to 65 percent within 500 hours. Theuseful UV output from the present lamp quartz envelope declines onlyabout 5 percent in 500 hours. This excellent maintenance factor cannotbe predicted from either low or higher pressure UV lamp experience ofthe past.

Of great importance in processes, such as printing, which do not allowlengthy down time is the rapid restarting characteristic of the presentUV lamp. In the event of a momentary loss or reduction of electricalpower the present lamp will restart in approximately one second on theaverage. The significantly higher output and faster restarting of thepresent lamp cannot be explained from experience with medium or highpressure UV lamps which typically require three to five minutes forrestarting, a down time which cannot be tolerated in photopolymeric inkcuring.

The present lamp compares most favorably with higher pressure lamps ineconomy of power and ballast. The present lamp is about one-third asintense as a medium pressure lamp up to 3,600 A, but requires aboutone-fourth the input power, and thus has a better input to output powerfactor. A standard ballast for a medium pressure UV lamp, however, costsapproximately ten times as much and weighs twenty times as much as aballast for operating the present lamp. Moreover the present lamp,having a low infrared emission, may be placed closer to print beingcured without thermal degradation of print or paper, and thus compensatefor its lower UV intensity while maintaining the substantial economy inpower and ballast cost and bulk.

The foregoing description is illustrative of the several unexpectedadvantages of the low pressure UV lamp of the present invention fallingwithin the scope of the ap pended claims.

I claim:

1. A low pressure ultraviolet lamp comprising:

a quartz envelope,

electron emissive electrodes spaced apart in the envelope, and a fill ofmercury and a mixture of neon and between 0.1 percent and 10 percentargon gas by volume at a pressure of approximately 7 torr or less,

the lamp having an input power density characteristic of approximatelynine watts per inch and maintaining an effective external ultravioletenergy output of over milliwatts per square inch at one meter.

has an exhaust spaced from the pressed portion.

2. A lamp according to claim 1 wherein a conductor through the envelopeto the electrode is embedded in a pressed portion of the envelope.
 3. Alamp according to claim 2 wherein the envelope has an exhaust spacedfrom the pressed portion.